Narwhals, nicknamed the "unicorns of the sea" because of their signature head tusks, exhibit an "alarming" response to human-caused stress that may lead to brain damage, researchers said Thursday.

When fearful, narwhals hold their breath while trying to swim away fast and deep, allowing their heart rates to drop from 60 beats per minute to three or four.

During escape dives, narwhals needed 97 percent their oxygen supply and often exceeded their aerobic dive limit, or "depletion of oxygen stores in the muscles, lungs, and blood, followed by anaerobic metabolism," said the study in the journal Science.

Normal dives of similar duration and depth used only about 52 percent of a narwhal's oxygen store, and heart rates dipped to about 20 beats per minute.

This combination of freezing while and entering a "flight or fight" response could make it hard for narwhals to get enough oxygen to the brain and other critical organs, researchers said.

The study has "cautionary" implications for narwhals and other whales, dolphins and marine life affected by human activities like shipping, seismic exploration and drilling for oil, said lead author lead author Terrie Williams, a professor of ecology and evolutionary biology at the University of California Santa Cruz.

"The biology of these animals makes them especially vulnerable to disturbance," she said.

Although narwhals are not endangered, they are increasingly coming in contact with humans as the planet warms and ice melts in their Arctic habitat.

To test their responses after being snared in nets set by native hunters, researchers fitted five narwhals with suction-cup sensors, much like Fitbit activity trackers, and monitored their physiological and behavioral responses.

They released the narwhals back into Scoresby Sound on the east coast of Greenland.

The sensors fell off within days and floated back to the surface, where researchers collected them.

"This technology has given us a window into the narwhal's world, and what we see is alarming," said Williams.

Previous research has shown that dolphins and seals also experience frequent heart arrhythmias when they swim fast in deep water, risking disorientation and death.

Narwhals' natural escape response -- to avoid killer whales and other threats -- usually involves a slow descent or ascent into an area where predators can't follow.

"Unlike threats from predators like killer whales, noise from sonar or a seismic explosion is difficult to escape," said Williams.

"The question is, what are we as humans going to do about it?"

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